1. Field of the Invention
This invention relates to nuclear measurements involving the spectroscopic analysis of energy spectra of gamma rays resulting from the interaction of neutrons with atoms of elements constituting an unknown material. The invention can find application in nuclear well logging techniques, wherein a sonde is lowered in a borehole and carries out spectral measurements from which is derived information about the composition of the earth formation surrounding the borehole, or the borehole fluid, or the annulus including casing and cement located between the borehole wall and the formation.
2. Related Art
A major goal of well logging is to obtain quantitative and qualitative information related to hydrocarbons in earth formation surrounding a borehole. A substantial part of nuclear well logging techniques are based on spectral analysis of energy spectra of gamma rays resulting from interactions of atoms with neutrons emitted from the sonde, such gamma rays being representative of certain atoms of the lithology (i.e. the matrix or the formation fluid) or of the borehole.
However, the gamma rays detected are representative of both the formation and the borehole. Thus, it is important to determine respective contributions of the borehole and the formation. In order to penetrate the formation, the fast neutrons must pass through the fluid contents of the borehole before entering the formation. The resulting borehole contributions to the gamma ray spectra significantly complicate the analysis of the formation composition. The problem is all the more complex since the sensivity of the detector(s) to the radiations coming from the borehole and the formation, is a function of many parameters, such as, to name a few, lithology, porosity, borehole size, casing size/weight/eccentricity, cement quality, or borehole fluid composition.
One way of accounting for these contributions is to calibrate the logging tool in a reference borehole having known borehole contents and formation compositions. However, this requires a large number of calibration measurements. Also laboratory conditions do not necessarily reflect the real composition of the contents of the borehole, so inaccuracies can result in the constituent proportions obtained from the spectra matching process. Taking more accurate account of the composition of an individual borehole's contents would enable more accurate information to be obtained concerning the constituents of the earth formations surrounding a borehole. Although the composition of the contents of the borehole may be determined with other logging tools, the use of the logs from such tools to correct the spectral analysis requires accurate recording of the measurements and of the corresponding positions along the borehole. Separate borehole passes may be required for each measurement, contributing further to errors which arise from merging the data to assure depth correspondence. Also, each additional log requires additional expense and delay and contributes further errors. Finally, some necessary data might simply not be available; for example, the borehole diameter is no longer available once a casing has been put in the well.
Another known way to assert the borehole influence on the measurements is to use two detectors having different sensitivities to gamma rays. One detector is preferentially sensitive to gamma rays coming from the formation, while the other preferentially receives gamma rays from the borehole. This differential sensitivity can be achieved e.g. by shielding (U.S. Pat. No. 4,937,446), and/or by specific geometrical configuration of the detectors (U.S. Pat. Nos. 4,721,853 or 4,445,033 or 3,890,501). Nevertheless, these mechanical arrangements, besides adding complexity to the logging sonde, cannot fully discriminate the borehole radiations from the formation radiations.
In case the gamma ray measurements are carried out with a single detector, one needs a reference, based on additional information, such as a prior knowledge of certain parameters and/or geometrical/physical configuration of the borehole under investigation. For instance, such reference can be based on calibrations in situ carried out in a known portion of the borehole and providing a value or values of a given parameter (e.g. water/oil fraction). However, improper selection or unavailability of a zone in the borehole having known characteristics can lead to incorrect parameter values and thus jeopardize the determination of the ultimate unknown(s).
In most gamma ray measurements, the energy spectra of gamma rays resulting from either the "capture" of thermal neutrons or the "inelastic" collisions of neutrons with atoms, after being decomposed into contributions due to individual atomic elements, usually called "elemental yields", reveal information concerning the presence of earth formations elements such as hydrogen, silicon, calcium, chlorine, sulfur and iron. Important petrophysical parameters can be derived from the elemental yields, such as porosity, matrix lithology, and water salinity. Examples of capture gamma ray spectra analysis are depicted in U.S. Pat. Nos. 3,521,064 to Moran et al., 4,464,569 to Flaum, 4,507,554 to Hertzog & Nelligan, 4,661,701 to Grau, 4,810,876 to Wraight et al. U.S. Pat. No. 4,937,446 to Roscoe, Stoller and McKeon shows an inelastic gamma ray spectral analysis. All the above mentioned patents are assigned to the assignee of the present application, and are as well incorporated herein by reference. An example of inelastic gamma ray spectroscopy is the so-called C/O measurement, the purpose of which is to determine oil and water saturation in the formation and in the borehole. The C/O measurement involves the determination of either the ratio of carbon to oxygen count rates in two respective energy windows (see U.S. Pat. No. 4,454,420) or the ratio of carbon and oxygen yields (see U.S. Pat. No. 4,937,446). The water saturation "S.sub.w " is determined from the C/O yields ratio, from the porosity PHI (known e.g. from other logs), and from external knowledge such as the lithology (from other logs) or the hydrocarbon fraction in the borehole (from calibrations in situ and preparation of the well comprising circulating known fluid in the well). Each measured C/O value is plotted against the known porosity PHI. The plot includes two sets of reference lines established in known configurations, with one set relating to a water filled borehole and another set to an oil filled borehole. Depending upon the position of the measured value on the plot with respect to the reference lines, one can derive S.sub.w. This interpretation technique, described in #14460 "Response of the Carbon/Oxygen Measurements for an Inelastic Gamma Ray Spectroscopy Tool" by B. A. Roscoe & J. A. Grau, presented at the 1985 SPE Annual Technical Conference and Exhibit held in Las Vegas, Sept. 22-25, 1985, relies heavily on information from other sources, which might not always be available or reliable.
It has been proposed, as depicted in U.S. Pat. No. 4,507,554 to Hertzog and Nelligan, assigned to the assignee of the present application, a method of determining the composition of the borehole material in which an early and a late spectrum of capture gamma rays are obtained in respective time periods following the neutron burst. The recorded spectra are analyzed using sets of standard spectra specific to each time period. It is assumed that the earlier of the two capture spectra contains information about both the borehole and the formation, whereas the later capture spectrum contains information only, or at least primarily, about the formation. Accordingly, the difference between the constituent analyses derived from the capture spectra is taken to indicate the composition of the borehole. This technique has the disadvantage that the time period between successive neutron bursts may be relatively long, to allow the radiation emanating from the borehole constituents to subside sufficiently before the second capture spectrum is recorded. Consequently, the logging speed must be relatively low, or alternatively poor depth resolution of the logs must be accepted. In addition, the assumption of little or no borehole contribution to the second capture spectrum is only an approximation, and thus does not necessarily reflect the real environment in which the spectral measurements are made.
Also, a method for correcting for the borehole effect in inelastic gamma ray spectroscopy has been described in SPE paper #14460 already referred to. The depicted method aims at determining the parameters upon which depend the carbon-to-oxygen ratio and is based on the assumption that porosity and lithology are both known.
The 446' Patent already referred to, describes a logging sonde designed for C/O yield measurement and provided with a near and a far detector. The relative amounts of carbon and oxygen C.sup.n, O.sup.n as measured from the near detector, and the relative amounts of carbon and oxygen C.sup.f, O.sup.f, as measured from the far detector, are obtained. A least squares analysis is performed to determine C.sup.n, O.sup.n from the energy spectrum (counts versus energy) acquired from the near detector, using standard spectra for the near detector. C.sup.f and O.sup.f are determined from the energy spectrum as measured from the far detector using standard spectra for the far detector. The analysis is performed at each logging depth in the borehole. Next, the carbon and oxygen determinations of the near and far detectors are combined to determine oil saturation of the formation (S.sub.o) and/or the oil percentage in the borehole (Y.sub.o). This is done by assuming that the total carbon and oxygen yields measured as indicated above are equal to the sum of the carbon and oxygen yields from the rock matrix of the formation, the pore space fluid, and the borehole fluid. The term "yield" means here the fractional number of gamma ray counts coming from a specific element. Carbon and oxygen yields may be expressed as a function of S.sub.o (oil saturation in the formation, or percentage of oil in the pore space) and Y.sub.o (the percentage of oil in the borehole), including coefficients which are determined under laboratory conditions by taking at least three measurements under the same conditions except for varying S.sub.o and Y.sub.o. Next, a carbon/oxygen ratio is formed for the near and the far detector respectively, i.e. C.sup.n /O.sup.n and C.sup.f /O.sup.f, leading to two equations which are solved for S.sub.o and Y.sub.o. At each depth in the borehole, a signal representative of oil saturation S.sub.o, and water saturation S.sub.w 1-S.sub.o, and percentage oil in the borehole Y.sub.o, is recorded.
Furthermore, capture gamma rays can be used for determining the porosity of the formation. In clean formations whose pores are filled with water or oil, the neutron log reflects the amount of liquid-filled porosity. High-energy neutrons (called fast neutrons), emitted from the sonde, collide with nuclei of the formation materials and, with each collision, lose a certain amount of their energy which depends on the relative mass of the nucleus with which the neutron collides. The greatest energy loss occurs when the neutron strikes a nucleus of practically equal mass, i.e. a hydrogen nucleus. Thus, the slowing of neutrons depends largely on the amount of hydrogen in the formation. Within a few microseconds, the neutrons have been slowed by successive collisions to thermal velocities, corresponding to energies of around 0.025 eV. They then diffuse randomly, without losing more energy, until they are captured by the nuclei of atoms such as chlorine, hydrogen, or silicon. The capturing nucleus becomes intensely excited and emits a high-energy capture gamma ray. Depending on the type of neutron logging sonde, either these capture gamma rays or the neutrons themselves are counted by a detector in the sonde. When the hydrogen concentration of the material surrounding the neutron source is large, most of the neutrons are slowed and captured within a short distance of the source. On the contrary, if the hydrogen concentration is small, the neutrons travel farther from the source before being captured. Accordingly, the counting rate at the detector increases for decreased hydrogen concentration, and vice versa. Examples of implementation of such method can be found in U.S. Pat. Nos. 4,816,674 to Ellis et al. or 4,423,323 to Ellis et al. both assigned to the assignee of the present application.
It has been proposed, as described in U.S. Pat. No. 4,788,424 commonly assigned with the present application, a method for producing an indication of the partition between the borehole and the formation of the constituents identified by detecting and counting capture gamma rays according to their energy in each of two time gates. The resulting energy spectra are analyzed to determine the type and relative gamma ray yield of each constituent of the borehole and formation. A characteristic neutron capture decay time constant for each constituent is derived from the yields and total gamma ray counts in the two time gates, and time constants for the borehole and formation overall are set equal to the derived time constants for constituents, such as iron and silicon, occurring predominantly in the borehole and formation respectively. The partition of the remaining constituents is then determined by considering the characteristic time constant for each constituent to be the sum of the time constants for the borehole and formation regions weighted by the proportion of that constituent in each region, the borehole and formation time constants being assumed the same for all constituents and the sum of the proportions being unity.
It can be understood from the above that compensation or correction for the effects of borehole on the measurements rely on the knowledge of the respective contributions of the borehole and the formation to the measurements. Although the determination of this contribution by either of the above mentioned known methods has proven to be relatively satisfactory in the past, there is a need for improvement, especially for any method which could be less dependent upon external information, such as laboratory calibrations, "in situ" calibrations or other logs. This need is all the more critical when the borehole environment is not sufficiently known (from other sources), or when the validity of a calibration is questionable.